The potent stimulant of abuse, methamphetamine [METH] is highly addictive and chronic administration of this drug can lead to tolerance to METH's effects. The goal of this proposal to elucidate the underlying mechanisms of tolerance to METH that provide a transient resistance to toxicity resultant to multiple highdose administration. Similar mechanisms may underlie the resistance of adolescent rodents to METH toxicity. Several factors have been identified that appear to be closely linked to the persistent DA deficits in the striatum caused by multiple administrations of high doses of METH. Utilizing evidence from selected markers (e.g. the dopamine transporter [DAT], vesicular monoamine transporter-2 [VMAT-2] function, DAT complex formation, glial cell reactions, heat shock protein expression, and nitrogen-dependent reactive species production), these factors appear to fall into two broad classifications with respect to their temporal relationship to METH exposure. These include: a first stage that persists for ~ 8 hours after the METH treatment, and 2) a second stage that appears to occur from ~24-48 hrs. Our studies will examine the overall hypothesis: Exposure to escalating doses of METH leads to a temporary alteration in mechanisms that trigger Stage 2 after Stage 1 expression, thereby preventing occurrence of METH-induced toxicity. The following Specific Aims are designed to prove or disprove this hypothesis: (I): Determine the effects and temporal profile of a tolerance-inducing METH pretreatment on nigrostriatal DA systems per se, and the response of Stages 1 and 2 to subsequent challenge with multiple high-dose METH administrations; (II) Determine the effects of a tolerance-inducing METH pretreatment on the expression of oxidatively linked systems per se, and their response in Stages 1 and 2 to subsequent challenge with multiple high-dose METH administrations. (Ill) Determine the effects of a tolerance-inducing METH pretreatment on DA, glutamate and glucocorticoid systems per se, and their response in Stages 1 and 2 to subsequent challenge with multiple high-dose METH administrations. The data obtained from the experiments in each of these Specific Aims will increase our understanding of the neurochemistry involved in the development of tolerance to METH. In addition, together with experiments in adolescent animals, these data will provide insight into issues related to drug abuse vulnerability in the adolescent population.